1. Field of the Invention
The present invention relates to the field of television sets and, more specifically, to the correction of the horizontal deviation of the electron beam of a cathode-ray tube during an image frame.
2. Discussion of the Related Art
In a cathode-ray tube, the image is obtained by a vertically scanning (for example at a frequency on the order of 50 kHz) and horizontally scanning (for example at a frequency on the order of 15.6 kHz) an electron beam on a phosphor screen. Since the screen is not spherical but rather planar, the scanning requires certain corrections to avoid image deformation.
The present invention more specifically applies to the so-called "pincushion" or "east/west" correction correcting the deformations on the left-hand and right-hand sides of the screen by modifying the horizontal scanning current during an image frame.
FIG. 1 very schematically shows a substantially rectangular screen 1 of a cathode-ray tube. This drawing illustrates the deformation undergone by an image 2 (hatched surface area in FIG. 1) in the absence of pincushion correction.
The pincushion correction is generally obtained by modulating the horizontal scanning current by an approximately parabolic signal of a duration substantially corresponding to the period of an image frame.
FIG. 2 shows, in the form of a block diagram, a conventional example of pincushion correction circuit.
The pincushion correction is obtained by modulating (MODUL. 3) the horizontal scanning current HB of cathode-ray tube 4 with an approximately parabolic signal p. This signal p is obtained by a so-called "Gilbert" multiplier G-MULT. 5, the function of which is to square (x.sup.2) a current ramp i.sub.ramp, symmetrical with respect to the ground and to the image frame frequency, and issued by a generator RAMP 6.
For certain cathode-ray tubes, and specifically for so called "wide-angle" (for example, on the order of 110.degree.) cathode-ray tubes for which the screen is closer to the electron gun than in a tube with a smaller angle, the image deformation is performed not only on the right-hand and left-hand edges, but also at the screen corners.
For this type of screen, a so-called "corner" correction which has the object of modifying the ends of parabola p in order to inflect them more or less significantly is also provided.
FIGS. 3A to 3C illustrate the functional principle of a conventional corner correction circuit. FIG. 3A partially shows screen 1 in a vertical orientation. FIG. 3B shows in dotted lines the outlook of signal p of FIG. 2 and, in full lines, the outlook of a correction signal p', obtained by subtracting to signal p, for example, a sinx signal (not shown). The three dotted lines in FIG. 3B and the three dotted lines in FIG. 3A are used to show the relationship between the instanteous magnitude of signal p (and for that matter signal p') and the position on the screen 1. That is, the points designated by the three vertical dotted lines (leftmost, centermost, rightmost) in FIG. 3B occur at the three positions respectively designated by the three vertical dotted lines (leftmost, centermost, rightmost) in FIG. 3A. FIG. 3C shows the outlook of current i.sub.ramp during an image frame period T. As illustrated in FIG. 3C, frame flyback interval RT between two image frames is used to cause the decrease of current i.sub.ramp until the following ramp. The cross-over point of the current i.sub.ramp, designated by the left-most dotted line in FIG. 3C, occurs at the same point as that designated in FIG. 3B by the longest of the three vertical dotted lines.
A disadvantage of a corner correction circuit consisting of adding or subtracting to parabola p in x.sup.2, a sinx function, in x.sup.4 or other, to obtain a parabola p', the ends of which have a slope CC (FIG. 3B) which is lower than that of parabola p, is that the added deformation also deforms the central portion CP of the parabola.
A difficulty of the corner correction is that it varies according to the cathode-ray tubes and is not, like the pincushion correction, substantially defined by a simple function (x.sup.2) independent from the tube involved. For a given type of tube, only the distances d on either side of the center h0 of the screen in the height h from which the slope of the ends of parabola p must be smaller are known, together with the amplitude of this slope reduction.
Document EP-A-0 561 043 describes a corner correction system in which the ends of the parabola are corrected after raising to the square of a linear ramp for obtaining this parabola.